This invention relates to fertilizer manufacturing, and more specifically, to the manufacture of urea-ammonium nitrate solutions wherein the ammonium nitrate is produced using urea process emissions.
In known processes for the production of fertilizers such as urea and/or ammonium nitrate, a substantial quantity of reaction by-product water is obtained from the urea process and from the nitric acid process unit associated with the ammonium nitrate process. This water can be distributed in three ways. Some of the water can be included in the urea-ammonium nitrate product solution, but the relative amount of water included in the product solution is usually constrained in the product specification to about 20%, being fixed by a requirement to obtain a solution containing 32% nitrogen. Another portion of the water can be sent to the absorption zone of the nitric acid process unit for use in the manufacture of nitric acid. Some of the water sent to the nitric acid plant is passed to the atmosphere being contained in the tailgas passed thereto. However, this amount of the water is very small, since it is proportional to the amount of nitrogen oxides passed in this tailgas, which must be controlled in order to comply with emission control standards. The remainder of the water can either be removed as a liquid from the ammonium nitrate process unit, or can be included in the tailgas passed from the condensing zone of the ammonium nitrate process unit to the atmosphere. When the ammonium nitrate process tailgas expelled to the atmosphere contains carbon dioxide, which would occur when crude tail gas emissions from a once-through type urea process unit is employed to provide ammonia as feed to the ammonium nitrate process unit, a fume can be created which has an opacity that may exceed the opacity level allowed by emission controls standards. The fume is comprised of particulate ammonium carbonate and/or ammonium bicarbonate, which form after emission of the ammonium nitrate tail gas to the atmosphere when it is expelled at a temperature above 60.degree. C. However, for the amount of condensate recovered in the condensing zone not to exceed the amount which can be recirculated for use elsewhere in the urea and/or ammonium nitrate processes, such as in the absorption zone of the nitric acid process unit, the tail gas expelled from the condensing zone must be at a temperature of no less than 80.degree. C. If the ammonium nitrate process tail gas is cooled below 80.degree. C., more condensate than can be used elsewhere in the processes is obtained. The excess condensate would then have to be treated and thereafter disposed of.
When the ammonium nitrate process tail gas is cooled to 60.degree. C. before emission of the tail gas to the atmosphere, particles of ammonium carbonate and/or ammonium bicarbonate form, which can be removed before the unreacted remainder of the carbon dioxide and ammonia contained in the tail gas is passed from the ammonium nitrate process unit to the atmosphere. However, as stated above, when the ammonium nitrate process tail gas is cooled to a temperature below 80.degree. C., excess water condenses out of the tail gas which cannot be recirculated for use in the process and must be disposed of.
The instant invention provides a process plant and a method for disposing of by-product water obtained from urea-ammonium nitrate processes such that emission controls standards can be met, and yet excess liquid waste effluent is avoided. Hence, treatment and subsequent disposal of the excess liquid effluent are not required.